The present invention relates to a process to make miniaturized multipolar flame-propagation-resistant cables having a reduced emission of toxic and noxious gases.
By the word "miniaturized", cables are intended in which the insulating layer thickness in the individual electrical conductors is included between 0.20 and 0.30 mm and the sheath thickness is included between 0.3 and 0.8 mm. Examples of miniaturized cables are the object of AMT 551070 specifications.
By the expression "flame-propagation-resistant" it is intended to mean that the cables assembled together to form bundles, must comply with the requirements established by CEI (Comitato Elettrotecnico Italiano, Italian Electrotechnical Committee) rules 20-22-III.
By the expression "reduced emission of toxic and noxious gases", it is intended to mean that the individual components of the cable when submitted to the tests established by CEI rule 20-37-II, give rise to an overall toxicity-index value of the cable, as hereinafter defined, lower than 3.5.
Said overall toxicity index of the cable is the sum of the toxicity indices of the individual components, each of them being multiplied by the ratio of the weight that each said component has in the cable unit of length to the overall weight that all the components have in the cable unit of length. The present invention also refers to the cables obtained by the process in question.
It is known that multipolar cables are cables provided, within one and the same sheath, with at least two and generally a plurality of electrical conductors which are individually insulated and assembled, being laid together for example.
The known process is comprised of the steps of:
combining together at least two and generally a plurality of electrical conductors which have been already individually insulated, i.e. already provided with an insulating layer of their own, said assembling being carried out for example by laying the conductors themselves together; PA1 inserting fillings into the gaps left between the conductors while they are being assembled, which fillings in the case of cables belonging to the flame-retardant cable class, are made of a practically fireproof material which therefore does not propagate flame, such as cables extruded from blends of polymeric materials highly charged with mineral fillers which, as such, do not propagate flame; and PA1 forming a sheath of a polymeric material about the assembly obtained by the preceding steps. PA1 combining together at least two electrical conductors, individually covered with an insulating layer, gaps being defined between said conductors combined together, PA1 inserting a filling into at least one fraction of said gaps, PA1 applying a sheath surrounding the assembly formed of the conductors combined together and the filling inserted in the gaps defined between said conductors, characterized in that the step of filling the gaps defined between the conductors comprises the steps of: PA1 at least two individually insulated electric conductors combined together, PA1 a filling inserted into the gaps existing between said insulated conductors combined together, PA1 a sheath surrounding the assembly formed of the insulated conductors combined together and the filling, characterized in that the filling inserted into the gaps between the insulated conductors comprises a blend of a first polymer selected from polydimethyl siloxanes having terminal vinyl groups, a second polymer selected from silicones containing Si--H groups and mineral fillers selected from magnesium hydroxide and aluminium hydroxide, in an amount included between 40% and 70% by weight of the overall weight of the blend.
While in known non-miniaturized multipolar low-voltage cables the conductor insulators have an average thickness of 0.82 mm, in miniaturized multipolar cables the insulator thickness is included between 0.20 and 0.30 mm on an average.
In the case of non-miniaturized cables no problem exists when polymeric material highly charged with mineral fillers is to be introduced by extrusion into the existing gaps between the assembled conductors. This is due to the fact that in non-miniaturized cables the thickness of the filling to be fitted into the gaps existing between the individual insulated conductors and around the assembly of same is of such a value that extrusion of the filling at relatively low temperatures is allowed without giving rise to discontinuities in the filling and/or important variations in the final diameter of the cable. On the contrary, the higher temperatures necessary for low-thickness (as in the case of miniaturized cables) extrusion of blends of polymeric materials highly charged with mineral fillers involves the presence of porosity in the filling itself caused by the emission of water vapour by desorption or decomposition of such hygroscopic mineral fillers.
It should be noted in fact that in order to be able to extrude, for example, a polyolefin-based blend containing mineral fillers such as magnesium hydroxide or aluminium hydroxide in an amount of 40% by weight with respect to 100 parts by weight of polymer, the temperature to be reached during the extrusion for making the blend fluid enough so that gaps between the conductors can be properly filled, shall be about 150.degree. C.
The Applicant has observed that the possibility of applying fillings formed of polymeric materials containing high amounts of mineral fillers by extrusion, is limited to a minimum thickness of 0.5 mm.
Therefore, the application of a filling by extrusion is to be excluded for miniaturized multipolar cables because in said cables the filling thickness between the conductors is on the order of 0.20-0.25 mm.
However, in order to be able to make miniaturized multipolar flame-propagation resistant cables it is necessary to carry out filling of the gaps between the assembled conductors by a material resisting flame propagation or flame-retardant material.
In a known solution it is provided that a glass rod or a glass-fibre cord is disposed into the gaps existing between the conductors combined together to form a cable.
This known solution, however, has some drawbacks. If glass rods combined with the cable conductors are used as the filling, the cable flexibility is clearly reduced. In addition, the glass rod's brittleness makes the arrangement of said rods close to the conductors troublesome.
If a glass-fibre cord is used as the filling, which cord may be optionally covered with a sheath of polymeric material, there is a risk that, due to breaking of some glass fibres in the cord, which fibres are very brittle being made of glass, said same glass fibres may project from the cord in the form of needles and consequently cause annoying injuries to the operators when they are assembling the cables with fittings such as connecting means or with appliances to be supplied power by the cable.
In both cases, in addition, since it is necessary to carry out coupling of the glass rods or glass-fibre cords, the assembling operations are made more complicated because the number of components to combine together is twice that of the insulated conductors.
Resorting to the use of section members of polymeric materials containing high amounts of mineral fillers in place of the glass rods or glass-fibre cords also involves the necessity, in addition to the complexity of the above mentioned assembling operation, to utilize section members having a very low tensile strength as compared with the tensile strength possessed by the insulated conductors, which will bring about the danger of breaking said section members while a cable is being manufactured.
A solution similar to the one disclosed in U.S. Pat. No. 4,978,649, comprises introducing, at room temperature, blends of polymers having a high flowability at room temperature and capable of crosslinking in time still at room temperature, into multipolar cables already provided with a sheath for creating fillings between the assembled conductors, does not seem to be practicable. In fact the addition of the amounts of mineral fillers necessary to make the miniaturized cable flame retardant to the blends designed to form the fillings gives rise to such viscosity values in said blends that they cannot be pumped at room temperature into the gaps existing between the conductors and sheath in a cable.